Lec. 02.Plant Water Relations

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Plant Water Relations Dr. K. Venkatesan Assoc. Prof. (CRP)

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All plant life requires water in large amount Present through out the plant body 80% is water & more than 90% in fresh weight Dormant seeds & buds – 10% WATER

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Occurs in 3 state Liquid Gas solid Colorless Odorless Tasteless

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Properties of water Has high specific heat Good conductor for heat / electricity Stabilizes temperature Solvent for electrolyte & non electrolyte Transparent to visible radiation Low viscosity High surface tension High latent heat of vaporization (540 cal g -1 ) – temperature buffer

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WATER AND HYDROGEN BONDS 1. Water consists of a single oxygen atom covalently bonded to two hydrogen atoms The oxygen atom is strongly electronegative so it attracts electrons 3. The electrons attracted are closer to the oxygen nucleus than the hydrogen nucleus so, the oxygen atom carries a partial negative charge and the hydrogen atoms carry a partial positive charge. This arrangement makes water a dipolar molecule

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4. The hydrogen bond is weaker than either a covalent or ionic bond. The hydrogen bond is responsible for the many unique properties of water a. Low melting temp.(0 o C) b. High boiling temp.(100 o C) c. High heat of vaporization (2452J/gm)

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pH pH = – log [H+] The pH scale is an inverse logarithmic representation of hydrogen proton (H+) concentration [H+]= 10 -1 M [H+]= 10 -9 M

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Role of water in plants Most abundantly used molecule Herbaceous plants 80 – 90 %; woody plants 50 % Cell turgidity - Structural frame work Cell enlargement & growth, Transpiration – Helps cooling leaf temperature (energy distribution through heat transfer) Helps gas exchange by stomatal opening Carrier for nutrients & metabolites

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Medium for biochemical reaction Solvent for nutrients & assimilates; gases, minerals & other solutes Reactant in many biochemical reactions (photosynthesis & Hydrolysis) End product in respiration Water is the major constituent of protoplasm

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Water in cells Cell wall – imbibed water 5 – 40 % Cytoplasm – mature cell – 5 – 10 % Vacuole – 50 – 80 % or more (Eucalyptus 50%; rhododendron 70%) Osmotic potential (-1.0 to -3.0 M Pa i.e. –10 to –30 bars) Bound water – low vapour pressure unfrozen at 0° C not function as solvent unavailable for physiological processes

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1. A membrane which allows both solvent and solute molecule to pass through - permeable (cell wall). 2. Membranes which allow only solvent molecules to pass through - Semi-permeable . (Egg membrane) 3. The biological membranes are not perfectly semi-permeable. They also allow passing solute molecules only up to a certain extent and are selective in nature. They are known, as selectively or differentially permeable membranes. (Plasma membrane) 4. Does not allow anything to pass through is impermeable membrane (Cuticle) Membranes are classified into 4 types

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Put one tablespoon of sugar into a glass of water and stir. Solute = sugar Solvent = water

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Diffusion movement of molecules or ions from a region of higher concentration to a region of lower concentration Move along a diffusion gradient due to their own kinetic energy Diffusion Multidirectional

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Diffusion

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Osmosis Diffusion of water through a differentially permeable membrane from a region of higher potential to a region of lower potential Osmotic potential – pressure required to prevent osmosis Turgor pressure – (pressure potential) develops against the cell wall Diffusion Pressure Deficit – (concentration of solvent molecule diffuse towards the solution under pressure

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HYPOTONIC If the concentration of water in the medium surrounding a cell is greater than that of the cytosol, the medium is said to be hypotonic . Water enters the cell by osmosis. When cells are placed in a solution, they neither gain nor lose water by osmosis. Such a solution is said to be isotonic. HYPERTONIC If cells are placed in higher concentration solution, they lose water by osmosis and the cells shrivel up. Such a solution is called hypertonic. ISOTONIC

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Osmotic effects on cells

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End osmosis living plant cell - hypotonic solution ( water)- O.P is lower than cell sap- water enters into the cell sap by osmosis- called end osmosis Eg: Dry resins in water Entry of water with the cell sap, a pressure is developed which press the protoplasm against the cell wall and become turgid. This pressure is called a turgor pressure Consequence of the turgor pressure is the wall pressure which is exerted by the elastic cell wall against the expanding protoplasm. At a given time, turgor pressure (T.P) equals the wall pressure (W.P) T.P = W.P

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Exosmosis Plant cell is placed in hypertonic solution (whose O.P is higher than cell sap- sugar solution) the water comes out the cell sap into the outer solution - cell becomes flaccid - exosmosis Eg: swollen grapes in 30 % sugar solution Cell (or) tissue will remain as such in isotonic solution

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Significance of osmosis in plants Absorption of water by roots from the soil Cell to cell movement of water and other substances Opening and closing of stomata depend upon the turgor pressure turgidity of the cells -shape or form of them organs Resistance of plants to drought and frost increases with increase in osmotic pressure to later cells Turgidity of the cells of the young seedling allows them to come out of the soil

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Water Potential Chemical potential of water Chemical potential of a substance is a measure of the capacity of that substance to do work. Water potential of pure water - 0 bars " " sea water – 28 bars at 25  C (1 bar = 0.1 M Pa) - 29 bars at 30 C 1 MPa = 9.87 atm = 10 bars (1 atm = 1.013 bar & 1 bar = 0.987 atm)

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Components of cell water potential  =  s +  p +  g +  m  = - miRT bars (psi – pounds per squire inch) m – isomolar concentration, i – ionisation constant T – absolute temperature (270 + room temp.) R- gas constant (0.083)  = -DPD (water potential is equal but opposite to the DPD)  =  s +  p Water potential ( ) = Diffusion Pressure Deficit (DPD)

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Imbibition – Absorption of water by hydrophilic colloids - Entry of water into solids and resultant swelling “Certain substances if placed in a particular liquid absorb it and swell up” Pieces of grass or dry wood or dry seeds or agar - placed in water - swell up volume increased - substances- - imbibants- process-imbibition certain force - attraction - between -imbibants and the imbibed substance

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Plasmolysis – loss of cell water protoplasm begins to shrinks or contract and separate from the cell wall assures a spherical form ( Incipient plasmolysis ) continuous loss of water-complete protoplasam – separate from cell wall-rounded in shape De Plasmolysis – plasmolysed cell placed in water, the process of endosmosis -Water enters - turgid - protoplasm again assumes it normal shape and position

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Plasmolysis

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A B C Plasmolysis

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Turgid = plant parts filled with water Wilted = lack of turgidity

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